1. Field of the Invention
This invention relates generally to a system and method for estimating a battery state of charge and, more particularly, to a system and method for estimating battery state-of-charge using a magnetic sensor that measures the change in magnetic susceptibility of one or more of the materials in the battery, and providing offset correction, temperature correction and current correction of a current output signal from the magnetic sensor.
2. Discussion of the Related Art
Electric vehicles are becoming more and more prevalent. These vehicles include hybrid vehicles, such as the extended range electric vehicles (EREV), that combine a battery and a main power source, such as an internal combustion engine, fuel cell systems, etc., and electric only vehicles, such as the battery electric vehicles (BEV). All of these types of electric vehicles employ a high voltage battery that includes a number of battery cells. These batteries can be different battery types, such as lithium-ion, nickel metal hydride, lead-acid, etc. A typical high voltage battery system for an electric vehicle may include a large number of battery cells or modules including several battery cells to meet the vehicle power and energy requirements, where each battery module may include a certain number of battery cells, such as twelve cells. The individual battery cells may be electrically coupled in series, or a series of cells may be electrically coupled in parallel, where a number of cells in the module are connected in series and each module is electrically coupled to the other modules in parallel. Different vehicle designs include different battery designs that employ various trade-offs and advantages for a particular application.
In order to maximize battery durability and provide useful range information to a driver of the vehicle, it is important to be able to accurately determine the state-of-charge (SOC) of the battery in an electric or hybrid vehicle. A common method for estimating the SOC of a battery is by measuring the open circuit or no load voltage across the battery. The open circuit voltage measurement is easy to obtain, but unfortunately may be prone to errors. Open circuit voltage error may be introduced by a voltage sensor itself, by a voltage sensing circuit in a controller, by sizing of electronics hardware, ND converters, filter gains, or by combinations of these and other factors.
Certain batteries suitable for electric vehicles, such as nickel metal hydride and some types of lithium ion batteries, produce a nearly constant open circuit voltage across most of the range of the state-of-charge of the battery. Complex models and algorithms may be employed to estimate the open circuit voltage under a load, and very precise measurements of the true open circuit voltage may be acquired when the battery is at rest, but the fact remains that very little information about state-of-charge can be ascertained from voltage in certain regions of operation. A weak dependence of open circuit voltage on state-of-charge can result in erroneous estimations of the remaining battery energy and driving range of the vehicle, and can also lead to over or under-utilization of the battery. However, these types of batteries are usually still highly desirable as power sources for electric hybrid vehicles because of their low mass, high power capability and larger energy storage capacity. Accordingly, for vehicles that employ battery types that have a nearly constant open circuit voltage across most of the range of the battery SOC, it is desirable to provide a system and method for determining the SOC of the battery that is not based on its open circuit voltage.
In one specific lithium ion battery design, the positive terminal plates of the individual cells are comprised of iron phosphate and the negative terminal plates are comprised of graphite, where it is the magnetic properties of the iron phosphate that changes as the cell SOC changes in a linear manner. Specifically, as the SOC of the battery cell increases, the magnetic susceptibility of the iron phosphate decreases. Because of this, it is known in the art to employ magnetic sensors to measure the SOC of these types of battery.
U.S Patent Application Publication No. 2010/0079145, titled System and Method to Determine the State of Charge of a Battery Using Magnetostriction to Detect Magnetic Response of a Battery Material, assigned to the assignee of this application and herein incorporated by reference, discloses a technique for determining battery SOC based on the magnetic response of the battery material. The '145 application discloses a magnetic sensor that is coupled to a battery cell, where the sensor includes a first coil and a second coil. An AC signal is provided to the first coil that induces a magnetic field within the battery cell, which in turn induces a current flow in the second coil. The current flow in the second coil is measured where the magnetic properties of the battery cell change in response to changes in battery SOC.